Study On Low Temperature Sintering And Performance Optimization Of Microwave Dielectric Ceramics With Lithium-based Rock Salt Structure

With the rapid development of modern microwave mobile communication technology,microwave dielectric ceramics have become a hot spot in the field of dielectric materials research in the global,among which low temperature co-fired ceramics?LTCC?has attracted the attention of many scientific and technological workers.LTCCs are required to have low sintering temperature,excellent microwave dielectric properties and co-fired with Ag electrodes of good electrical conductivity.Li-containing rock-salt structured system ceramics have been widely reported because of its ultra-low dielectric loss and cheap as well as easy access for raw materials,but the excessive sintering temperature has limited its application in LTCC technology to some extent.In this article,two kinds of solid solution compounds microwave dielectric ceramics Li₅Ti₂O₆F and Li₇Ti₃O₉F with intrinsic sintering temperature below 960?were synthesized by solid solution reaction in rock salt structure low melting point LiF and Li₂TiO₃?In addition,we investigated the effect of different contents of LiF doping on Li₂Mg₂TiO₅ ceramics.After that,Sr₃?VO₄?₂ ceramics was used for the first time to adjust the temperature coefficient of resonant frequency of the Li2Mg2TiO5 ceramics.The phase composition,crystal structure,microstructure,spectroscopic properties and microwave dielectric properties of the above ceramics were characterized by XRD?FE-SEM?EDS?TEM?TG-DSC?XPS?far infrared reflectance spectroscopy?FIR?,Raman spectroscopy and vector network analyzer.The main contents are listed as follows:?1?Li₅Ti₂O₆F samples were prepared by conventional solid-state reaction method.XRD and DSC analysis show that the ceramic was undergo a phase transformation from monoclinic structure with a space point group of C2/c to cubic structure with a space point group of Fm-3m near 800?.The results of XRD refinement,SEAD and HRTEM further confirm that the crystal structure of the ceramic is cubic after phase transformation.The results of EDS elemental scanning indicate that LiF enters into the lattice of Li2TiO3.Li5Ti2O6F ceramics can be densified at lower temperature?880?,showing excellent microwave dielectric properties:?_r=19.6,Q×f=79,500GHz,?_f=-29.6 ppm/?,and has good chemical compatibility with Ag.The intrinsic dielectric constant of Li₅Ti₂O₆F ceramic is 19.4 and the quality factor is 97,000 GHz were obtained by fitting the far infrared reflectivity spectra.?2?Li₇Ti₃O₉F samples were prepared by conventional solid-state reaction method.XRD shows that the ceramic is cubic rock-salt structure in the temperature range875?975?,and the results of SAED and HRTEM also verify the above views.The results of EDS element scanning are similar to those of Li5Ti2O6F ceramics,indicating that the Li7Ti3O9F is an oxyfluoride solid solution compound.The Gaussian-Lorentz fitting of the XPS Ti-2p region shows that the trivalent Ti elements caused by oxygen vacancy at high temperature account for only 2%.The microwave dielectric properties of the Li7Ti3O9F ceramic at the optimum sintering temperature of 950?are as follows:?_r=22.5,Q×f=88,200 GHz,?_f=-24.2 ppm/?.No chemical reaction occurs in co-firing with the Ag electrode,indicating that it can be applied in LTCC technology.The average thermal expansion coefficient of ceramic from room temperature to 800?is 11.2 ppm/?.The broadening Raman spectra of Li₇Ti₃O₉F also confirms its long-range disordered arrangement,which is significantly different from the pure Li2TiO3 of monoclinic structures.The results of far-infrared reflectivity spectra show that the polarization of ceramic at microwave frequency is mainly derived from the absorption of phonon oscillations in the infrared region.?3?In order to solve the remarkable porous microstructure and low relative density of Li₂Mg₂TiO₅ ceramics at elevated temperatures,this chapter aims to further improve the microwave dielectric properties by using the low melting point LiF to reduce the sintering temperature while using its liquid phase sintering mechanism.A series of Li₂Mg₂TiO₅+n wt%LiF?1?n?5?ceramics were prepared to investigate the effect of Li F content on phase composition,microstructure and microwave dielectric properties systematically.As a result,the addition of 4 wt%LiF not only significantly reduced the densification temperature of the ceramics,but also improved the microstructure and enhanced the microwave performance.The microwave dielectric properties of Li₂Mg₂TiO₅+4 wt%LiF ceramics at 900?are?_r=14.6,Q×f=128,500 GHz,?_f=-35.6 ppm/?.Considering its relatively large negative?_f,we counteracted to near zero by Sr₃?VO₄?₂ for the first time.The composites0.65[Li₂Mg₂TiO₅+4 wt%LiF]-0.35[Sr₃?VO₄?₂+1 wt%B₂O₃]ceramics sintered at900?has a remarkable combination microwave properties:?_r=13.4,Q×f=91,860GHz,?_f=-1.6 ppm/?.Moreover,the composite ceramics has good chemical compatibility with silver and has a potential application value in LTCC.